Method and apparatus for compensating for the frequency offset in interleaved frequency division multiple access

ABSTRACT

Provided are a method and apparatus for compensating for a frequency offset in an interleaved frequency division multiple access. The method compensates for a frequency offset between a transmission signal and a reception signal for a u th  user (1≦u≦U, where U denotes the number of users) in an interleaved frequency division multiple access. The method includes: (a) estimating the frequency offset from a selection signal that is determined as the reception signal for the u th  user in an initial mode and as a feedback signal in a normal mode; (b) estimating multiple access interferences representing an extent to which reception signals for i th  other users (1≦i≦U−1) at the same time interfere with the reception signal for the u th  user; (c) subtracting the estimated multiple access interferences from the reception signal for the u th  user and determining the subtraction result as the feedback signal; (d) determining whether steps (a), (b), and (c) have been repeated a predetermined number of times, and if it is determined that steps (a), (b), and (c) have not been repeated the predetermined number of times, going back to step (a); and (e) if it is determined that steps (a), (b), and (c) have been repeated the predetermined number of times, estimating the transmission signal for the u th  user using the feedback signal finally determined in step (c) and the estimated frequency offset.

This application claims the priority of Korean Patent Application No.2002-44461, filed on Jul. 27, 2002, in the Korean Intellectual PropertyOffice, the contents of which is incorporated herein in its entirety byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a frequency division multiple access,and more particularly, to a method and apparatus for compensating forthe frequency offset in an interleaved frequency division multipleaccess.

2. Description of the Related Art

The frequency offset in an interleaved frequency division multipleaccess (IFDMA) communication device decreases a signal-to-noise ratio(SNR) by changing the magnitude and phase of a signal that istransmitted from a transmitter to a receiver and creates interferencesamong users. IFDMA refers to a method of realizing multi-carriers in atime domain, which was disclosed in the paper of “Interleaved FDMA-A NewSpread Spectrum Multiple Access Scheme”, by Uli Sorger, Isabellar deBroeck, and Michael Schnell 1998 in International Conference oncommunication (ICC) proceedings, 1998, pp. 1013-1017. Here, thefrequency offset is caused by a difference between oscillationfrequencies generated by a transmission oscillator of a transmitter ofthe IFDMA communication device and a reception oscillator of a receiverthereof. To solve this, i.e., to minimize the frequency offset, verysophisticated analog radio frequency (RF)/intermediate frequency (IF)components should be used in the transmitter and the receiver of theIFDMA communication device. However, it is difficult to realize analogRF/IF components satisfying desired performances as the frequency offsetincreases. Also, although the analog RF/IF components are realized, themanufacturing cost thereof is high.

SUMMARY OF THE INVENTION

The present invention provides a method of compensating for theinfluence due to frequency offset in an IFDMA instead of removing thefrequency offset without using the characteristics of channels.

The present invention also provides an apparatus for compensating forthe frequency offset in an IFMDA to perform the method.

According to an aspect of the present invention, there is provided amethod of compensating for a frequency offset between a transmissionsignal and a reception signal for a u^(th) user (1≦u≦U, where U denotesthe number of users) in an interleaved frequency division multipleaccess. The method includes: (a) estimating the frequency offset from aselection signal that is determined as the reception signal for theu^(th) user in an initial mode and as a feedback signal in a normalmode; (b) estimating multiple access interferences representing anextent to which reception signals for i^(th) other users (1≦i≦U−1) atthe same time interfere with the reception signal for the u^(th) user;(c) subtracting the estimated multiple access interferences from thereception signal for the u^(th) user and determining the subtractionresult as the feedback signal; (d) determining whether steps (a), (b),and (c) have been repeated a predetermined number of times, and if it isdetermined that steps (a), (b), and (c) have not been repeated thepredetermined number of times, returning to step (a); and (e) if it isdetermined that steps (a), (b), and (c) have been repeated thepredetermined number of times, estimating the transmission signal forthe u^(th) user using the feedback signal finally determined in step (c)and the estimated frequency offset.

According to another aspect of the present invention, there is providedan apparatus for compensating for a frequency offset between atransmission signal and a reception signal for a u^(th) user (1≦u≦U,where U denotes the number of users) in an interleaved frequencydivision multiple access. The apparatus includes: a main frequencyoffset estimator, an extent estimator, a subtractor, a controller, and atransmission signal estimator. The main frequency offset estimatordetermines the reception signal for the u^(th) user or a feedback signalas a selection signal in response to a first control signal, estimatesthe frequency offset from the selection signal, and outputs theestimated frequency offset. The extent estimator estimates multipleaccess interferences representing an extent to which reception signalsfor from i^(th) other users (1≦i≦U−1) interfere with the receptionsignal for the u^(th) user, from the reception signals for the i^(th)other users, the selection signal, and the estimated frequency offset,and outputs the estimated multiple access interferences. The subtractorsubtracts the estimated interferences from the reception signal for theu^(th) user and outputs the subtraction result as the feedback signal.The controller generates the first control signal in response to theresult obtained by analyzing the state of the apparatus for compensatingfor the frequency offset, checks whether a predetermined period of timehas elapsed, and outputs a second control signal in response to thechecked result. The transmission signal estimator estimates thetransmission signal for the u^(th) user from the feedback signal finallyinput from the subtractor and the estimated frequency offset in responseto the second control signal and outputs the estimated transmissionsignal. It is preferable that the main frequency offset estimator, theextent estimator, and the subtractor are enabled in response to thesecond control signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present inventionwill become more apparent by describing in detail exemplary embodimentsthereof with reference to the attached drawings in which:

FIG. 1 is a flowchart for explaining a method of compensating for thefrequency offset in an IFDMA according to the present invention;

FIG. 2 is a block diagram of an IFDMA communication device having afrequency offset compensating apparatus according to the presentinvention;

FIG. 3 is a block diagram of the frequency offset compensating apparatusaccording to the present invention;

FIG. 4 is a block diagram of a preferred embodiment of the presentinvention of a main frequency offset estimator shown in FIG. 3;

FIG. 5 is a block diagram of a preferred embodiment of the presentinvention of a transmission signal estimator shown in FIG. 3;

FIG. 6 is a block diagram of a preferred embodiment of the presentinvention of an extent estimator shown in FIG. 3;

FIG. 7 is a block diagram of a preferred embodiment of the presentinvention of an i^(th) sub frequency offset estimator shown in FIG. 6;

FIG. 8 is a block diagram of a preferred embodiment of the presentinvention of an i^(th) extent estimator shown in FIG. 6;

FIG. 9 is a block diagram of a preferred embodiment of the presentinvention of a feedback signal generator shown in FIG. 6;

FIG. 10 is a graph for illustrating the constellation of an ideallyreception signal that does not have a frequency offset;

FIG. 11 is a graph for illustrating the constellation of a receptionsignal when a frequency offset accounts for 3% of the distance amongsub-carriers;

FIG. 12 is a graph for illustrating the constellation of a receptionsignal when the method and apparatus of the present invention are usedto compensate for the frequency offset in the reception signal shown inFIG. 11;

FIG. 13 is a graph for illustrating a SNR versus the frequency offset;and

FIG. 14 is a graph for illustrating a bit error rate (BER) versus thefrequency offset.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, a method of compensating for frequency offset in an IFDMAaccording to the present invention will be described with reference tothe attached drawings.

FIG. 1 is a flowchart for explaining a method of compensating forfrequency offset in an IFDMA according to the present invention. Themethod is composed of steps 10 and 12 of estimating a frequency offsetand an interference, step 14 of obtaining a feedback signal, and step 16and 18 of estimating a transmission signal.

During IFDMA communications, the frequency offset compensating methodaccording to the present invention shown in FIG. 1 is used to compensatefor a frequency offset between a transmission signal and a receptionsignal for a u^(th) user (1 u U, where U denotes the number of users)and estimates a transmission signal from a reception signal. Here, thefrequency offset is caused by a difference between oscillationfrequencies generated by a transmission oscillator used in a transmitterto generate the transmission signal and a reception oscillator used in areceiver to generate the reception signal.

The frequency offset compensating method according to the presentinvention performs step 10 of estimating a frequency offset {circumflexover (ε)}{circumflex over (ε_(u))} for the u^(th) user from a selectionsignal. Here, a signal received in an initial mode where the frequencyoffset compensating method according to the present invention starts isdetermined as the selection signal, and then a feedback signal isdetermined as the selection signal in a normal mode. The frequencyoffset {circumflex over (ε)}{circumflex over (ε_(u))} can be estimatedfrom the selection signal using Equation 1:

$\begin{matrix}{\hat{ɛ_{u}} = \frac{\angle\; C_{u}}{2\;\pi}} & (1)\end{matrix}$wherein

${{\hat{ɛ_{u}}} < \frac{1}{2}},$and ∠C_(u), denotes the angle of a correlation value C_(u) which can becalculated using Equation 2:

C u = ∑ k = 1 N u - 1 ⁢ z k + N ( u ) ⁡ [ u ] ( z k ( u ) ⁡ [ u ] ) * ( 2 )wherein N_(u) denotes the number of carriers used by the u^(th) user, k(k=0, 1, . . . , N_(u)−1) denotes the position of the reception signalfor the u^(th) user among a plurality of reception signals contained ina frame, N denotes the number of chips constituting a block,

z k ⁢ + N ( u ) ⁡ [ u ]denotes the result obtained by delaying the selection signal

z k ( u ) ⁡ [ u ]by N, and

( z k ( u ) ⁡ [ u ] ) *denotes a conjugate of the selection signal

z k ( u ) ⁡ [ u ] .Here, the selection signal

z k ( u ) ⁡ [ u ]is determined as the reception signal for the u^(th) user in the initialmode and expressed by Equation 3:

$\begin{matrix}{r_{k}^{\lbrack u\rbrack} = {z_{k}^{{(u)}{\lbrack u\rbrack}} + {\sum\limits_{{i = 1},{i \neq u}}^{U}z_{k}^{{(i)}{\lbrack u\rbrack}}} + n_{k}^{\lbrack u\rbrack}}} & (3)\end{matrix}$wherein r_(k) ^([u]) denotes the reception signal for the u^(th) user,n_(k) ^([u]) denotes noise components contained in the reception signalr_(k) ^([u]) for the u^(th) user, and z_(k) ^((i)[u]) denotes the extentby which the reception signals for i^(th) (1 i U−1) other usersinterfere with the reception signal r_(k) ^([u]) for the u^(th) user.

After step 10, the reception signals for the i^(th) users interfere withthe reception signal r_(k) ^([u]) for the u^(th) user is estimated instep 12. Here, a multiple access interference

z k ( i ) ⁡ [ u ]indicating the extent by which one of the i^(th) users interferes withthe u^(th) user can be estimated using Equation 4:

z k ( i ) ⁡ [ u ] = { ⅇ j ⁢ ⁢ π ⁡ [ Δ i ⁢ ⁢ u ⁡ ( 2 ⁢ k / N - 1 / L u ) + ɛ i ^ ⁡( 1 / L i - 1 / L u ) ] ⁢ q i ⁢ sin ⁡ ( π ⁢ ɛ i ^ / L i ) L i ⁢ sin ⁡ [ π ⁡ ( Δi ⁢ ⁢ u + ɛ i ^ ) / L u ] · z k ⁢ % ⁢ N i ( i ) ⁡ [ i ] , for ⁢ ⁢ N i ≤ N u ⅇ j⁢⁢π ⁡ [ Δ i ⁢ ⁢ u ⁡ ( 2 ⁢ k / N - 1 / L i ) ] ⁢ q i ⁢ sin ⁡ ( π ⁢ ɛ i ^ / L i ) L i⁢sin ⁡ [ π ⁡ ( Δ i ⁢ ⁢ u + ɛ i ^ ) / L i ] · ∑ 1 = 0 M i ⁢ ⁢ u - 1 ⁢ ⅇ j2 ⁢ ⁢ π ⁡ (Δ i ⁢ ⁢ u + ɛ i ^ ) ⁢ 1 / L u ⁢ z k + 1 ⁢ N u ( i ) ⁡ [ i ] , for ⁢ ⁢ N i > N u( 4 )wherein

z k ( i ) ⁡ [ u ]denotes an estimated value of the multiple access interference

(z_(k)^((i)[u])),_(iu) denotes n_(i)-n_(u), n_(i) denotes a frequency offset assigned tothe i^(th) user, n_(u) denotes a frequency offset assigned to the u^(th)user, L_(u) denotes the number of times user symbols are repeated in atransmitter for the u^(th) user, L_(i) denotes the number of times usersymbols are repeated in a transmitter for the i^(th) user, {circumflexover (ε)}{circumflex over (ε_(i))} denotes a frequency offset of thereception signal for the i^(th) user, q_(i) denotes an initial phaseoffset of an i^(th) block, k % N_(i) denotes the remainder when k isdivided by N_(i), N_(i) denotes the number of carriers used by thei^(th) user, and

$M_{i\; u} = {\frac{N_{i}}{N_{u}} = {\frac{L_{u}}{L_{i}}.}}$Here, the frequency offsets n_(i) and n_(u) are different from thefrequency offset to be compensated for according to the presentinvention.

Accordingly, the sum of multiple access interferences

z k ( i ) ⁡ [ u ]that is, the extent by which the reception signals for U users, fromwhich the reception signal for the u^(th) user is excluded, interferewith the reception signal for the u^(th) user can be determined as atotal interference and be expressed by Equation 5:

∑ i = 1 , i ≠ u U ⁢ z k ( i ) ⁡ [ u ] ( 5 )

After step 12, in step 14, the interference is subtracted from thereception signal r_(k) ^([u]) for the u^(th) user using Equation 6, andthen the subtraction result is determined as a feedback signal

z k ( u ) ⁡ [ u ]that can be the selection signal in the normal mode as previouslydescribed.

z k ( u ) ⁡ [ u ] = r k [ u ] - ∑ i = 1 , i ≠ u U ⁢ z k ( i ) ⁡ [ u ] ( 6 )

After step 14, in step 16, whether steps 10, 12, and 14 have beenrepeated a predetermined number of times is determined. Here, thepredetermined number of times is determined in proportion to a reductionrate of SNR. In other words, the predetermined number of times can beincreased with a reduction in the SNR.

If it is determined that steps 10, 12, and 14 have not been repeated apredetermined number of times, the process goes to step 10, and thensteps 10, 12, and 14 are repeated. Here, in the frequency offsetcompensating method according to the present invention, a selectionsignal when step 10 is initially performed is determined as a receptionsignal, while a selection signal selected when step 10 is repeated isdetermined as a feedback signal not as a reception signal. However, ifin step 16 it is determined that steps 10, 12, and 14 have been repeatedthe predetermined number of times, in step 18 a transmission signaly_(k) ^([u]) is estimated from a feedback signal

z k ( u ) ⁡ [ u ]that is finally determined in step 14 and a frequency offset {circumflexover (ε)}{circumflex over (ε_(u))} that is finally determined in step10, and then the estimated transmission signal

y k [ u ]is obtained using Equation 7:

y k [ u ] = ⅇ - j ⁢ ⁢ π ⁡ [ Δ i ⁢ ⁢ u ⁡ ( 2 ⁢ k / N - 1 / L u ) + 1 ] ⁢ L u ⁢ sin⁡( π ⁢ ɛ u ^ / L u ) q u ^ ⁢ sin ( π ⁢ ⁢ ɛ u ^ ) ⁢ z k ( u ) ⁡ [ u ] ( 7 )wherein q_(u) denotes an initial phase offset in a u^(th) block and{circumflex over (q)}{circumflex over (q_(u))} denotes an estimationvalue of the initial phase offset q_(u).

The structure and operation of an IFDMA communication device having afrequency offset compensating apparatus, according to the presentinvention, for performing the method of compensating for the frequencyoffset in an IFDMA according to the present invention will be describedwith reference to the attached drawings.

FIG. 2 is a block diagram of an IFDMA communication device having afrequency offset compensating apparatus according to the presentinvention. Referring to FIG. 2, the IFDMA communication device includesfirst, . . . , u^(th), . . . , and U^(th) transmitters 30, . . . , 32, .. . , and 34 for first through U^(th) users, first, . . . , u^(th), . .. , and U^(th) channels 40, . . . , 42, . . . , and 44, an adder 46, andfirst, . . . , u^(th), . . . , and U^(th) receivers 50, . . . , 52, . .. , and 54 for the first through U^(th) users.

The first transmitter 30 shown in FIG. 2 includes a L₁ repeater 60, amultiplier 62, a first cycle prefix (CP) inserter 64, and a multiplier66. The u^(th) transmitter 32 includes an L_(u) repeater 70, amultiplier 72, an u^(th) CP inserter 74, and a multiplier 76. The U^(th)transmitter 34 includes a L_(u) repeater 80, a multiplier 82, a U^(th)CP inserter 84, and a multiplier 86.

Here, the L₁ repeater 60 repeats a user symbol α_(k) ^((l)) for thefirst user L₁ times and then outputs the repeated user symbol to themultiplier 62. The L_(u) repeater 70 repeats a user symbol α_(k) ^((u))for the u^(th) user L_(u) times and then outputs the repeated usersymbol to the multiplier 72. The L_(u) repeater 80 repeats a user symbolα_(k) ^((U)) for the U^(th) user L_(U) times and then outputs therepeated user symbol to the multiplier 82. Here, the user symbol α_(k)^((l)) is composed of N₁ quadrature amplitude modulation (QAM) encodedsignals, the user symbol α_(k) ^((u)) is composed of N_(u) QAM encodedsignals, and the user symbol α_(k) ^((U)) is composed of N_(U) QAMencoded signals.

The multipliers 62, 72, and 82 respectively multiply the repetitionresults output from the L₁, L_(u), and L_(U) repeaters 60, 70, and 80 by

${\alpha_{1}{\mathbb{e}}^{\frac{{j2}\;\pi\; n_{1}k}{N}}},$

${\alpha_{u}{\mathbb{e}}^{\frac{{j2}\;\pi\; n_{u}k}{N}}},$and

$\alpha_{U}{\mathbb{e}}^{\frac{{j2}\;\pi\; n_{U}k}{N}}$and then output the multiplication results to the first, u^(th), andU^(th) CP inserters 64, 74, and 84.

The first, u^(th), or U^(th) CP inserter 64, 74, or 84 removesinterferences among blocks, inserts a CP before the multiplicationresult output from the multiplier 62, 72, or 82, and outputs theinsertion result to the multiplier 66, 76, or 86.

In order to convert the insertion results into an RF signal, themultipliers 66, 76, and 86 multiply the insertion results output fromthe first, u^(th), and U^(th) CP inserter 64, 74, and 84 by e^(j2π∫) ⁰^(t) and then output the multiplication results to the first, . . .u^(th), . . . , and U^(th) channels 40, . . . , 42, . . . , and 44Transmission signals y_(k) ^((l)), . . . , y_(k) ^((u)), . . . , andy_(k) ^((U)) that have passed through the first, . . . u^(th), . . . ,and U^(th) channels 40, . . . , 42, . . . , and 44 are added and thenthe addition result is added to noise by the adder 46. Here, the adder46 is not an actually existing device but is shown to conceptionallyexplain that noise factors into the addition result.

The first receiver 50 shown in FIG. 2 includes a multiplier 90, a firstCP remover 92, a multiplier 94, a first overlapper/adder 96, a frequencyoffset compensating apparatus 98, and a first equalizer and checker 100.The u^(th) receiver 52 includes a multiplier 110, an u^(th) CP remover112, a multiplier 114, an u^(th) overlapper/adder 116, a frequencyoffset compensating apparatus 118, and an u^(th) equalizer and checker120. The U^(th) receiver 54 includes a multiplier 130, an U^(th) CPremover 132, a multiplier 134, an U^(th) overlapper/adder 136, afrequency offset compensating apparatus 138, and an U^(th) equalizer andchecker 140.

In order to convert the RF signal, i.e., the addition result, into abase band signal, the multipliers 90, 110, and 130 multiply the additionresult obtained by the adder 46 by e^(−j2×{circumflex over (f)}) ⁰ ^(t)and then output the multiplication results to the first, u^(th), andU^(th) CP removers 92, 112, and 132, respectively. Here, a differencebetween a frequency f₀ generated by the transmission oscillator and afrequency {circumflex over (f)}{circumflex over (f₀)} generated by thereception oscillator corresponds to the frequency offset that is to becompensated for according to the present invention.

Here, the first, u^(th), and U^(th) CP remover 92, 112, or 132 removes aCP from the multiplication result obtained by the multiplier 90, 110, or130 and then outputs the removal result to the multiplier 94, 114, or134. The multipliers 94, 114, and 134 multiply the removal resultsoutput from the first, u^(th), and U^(th) CP removers 92, 112, and 132by

${\mathbb{e}}^{\frac{{- {j2}}\;\pi\; n_{1}k}{N}},$

${\mathbb{e}}^{\frac{{- {j2}}\;\pi\; n_{u}k}{N}},$and

${\mathbb{e}}^{\frac{{- {j2}}\;\pi\; n_{U}k}{N}}$and then output the multiplication results to the first, u^(th), andU^(th) overlappers/adders 96, 116, and 136.

The first, u^(th), and U^(th) overlappers/adders 96, 116, and 136overlap the multiplication results output from the multipliers 94, 114,and 134 for N₁, N_(u), and N_(U) cycles, add the overlap results, andoutput the addition results to the frequency offset compensatingapparatuses 98, 118, and 138. For example, the u^(th) overlapper/adder116 may output the reception signal r_(k) ^((u)) calculated usingequation 3 to the frequency offset compensating apparatus 118.

The frequency offset compensating apparatus 98, 118, or 138 shown inFIG. 2 performs the frequency offset compensating method according tothe present invention shown in FIG. 1 to estimate the transmissionsignal y_(k) ^((l)), y_(k) ^((u)), or y_(k) ^((U)), and to output theestimated transmission signal

y k [ 1 ] , y k [ u ] ,or

y k [ U ] ]to the first, u^(th), or U^(th) equalizer and checker 100, 120, or 140.

The first, u^(th), and U^(th) equalizer and checkers 100, 120, and 140equalize the estimated transmission signals

y k [ 1 ] , y k [ u ] ,and

y k [ U ]that are output from the frequency offset compensating apparatuses 98,118, and 138 and have inter-symbol interferences (ISIs) caused by thefirst, . . . , u^(th), . . . , and U^(th) channels 40, . . . , 42, . . ., and 44 to remove the ISIs from the estimated transmissions signals

y k [ 1 ] , y k [ u ] ,and

y k [ U ] ,estimate the user symbols α_(k) ^((l)), α_(k) ^((u)), and α_(k) ^((U)),and output estimated user symbols

a k ( 1 ) , ⁢ a k ( U ) ,and

a k ( 1 ) .

The structure and operation of the frequency offset compensatingapparatus 118 for performing the method of compensating for frequencyoffset in an IFDMA according to the present invention will be describedwith reference to the attached drawings. Here, the frequency offsetcompensating apparatuses 98 and 138 shown in FIG. 2 have the samestructure and perform the same operation as the frequency offsetcompensating apparatus 118.

FIG. 3 is a block diagram of the frequency offset compensating apparatusaccording to the present invention. The frequency offset compensatingapparatus includes a main frequency offset estimator 160, aninterference estimator 162, a subtractor 164, a transmission signalestimator 166, and a controller 168.

In order to carry out step 10 of FIG. 1 the main frequency offsetestimator 160 shown in FIG. 3 determines the reception signal r_(k)^([u]) and the feedback signal z_(k) ^({circumflex u])} (()}û) as theselection signal in response to a first control signal C1 input from thecontroller 168, estimates the frequency offset {circumflex over(ε)}{circumflex over (ε_(u))} of the u^(th) user from the selectionsignal as expressed by Equation 1, and outputs the estimated frequencyoffset {circumflex over (ε)}{circumflex over (ε_(u))} to theinterference estimator 162 and the transmission signal estimator 166.For this, the controller 168 generates the first control signal C1 inresponse to the result obtained by analyzing the state of the frequencyoffset compensating apparatus shown in FIG. 3 and then outputs the firstcontrol signal C1 to the main frequency offset estimator 160. Forexample, the controller 168 generates the first control signal C1 andthen outputs the first control signal C1 to the frequency offsetestimator 160, so that the main frequency offset estimator 160determines the reception signal r_(k) ^([u]) when the frequency offsetcompensating apparatus shown in FIG. 3 is in the initial mode as theselection signal and then determines the feedback signal z_(k)^({circumflex u])} (()}û) when the frequency offset compensatingapparatus is in the normal mode as the selection signal.

FIG. 4 is a block diagram of a preferred embodiment 160A of the presentinvention of the main frequency offset estimator 160 shown in FIG. 3.The main frequency offset estimator 160A includes a first selector 180,a first delayer 182, a first conjugate calculator 184, a firstmultiplier 186, and a first offset calculator 188.

The first selector 180 of the main frequency offset estimator 160A shownin FIG. 4 selects one of the feedback signal z_(k)^({circumflex u])} (()}û) input from the subtractor 164 and thereception signal r_(k) ^([u]) input from the outside in response to thefirst control signal C1 input from the controller 168 and then outputsthe selection result as the selection signal to the first delayer 182and the first conjugate calculator 184 as well as via an output portOUT2. For example, the first selector 180 determines the receptionsignal r_(k) ^([u]) from input from the outside as the selection signaland then outputs the selection signal if it is determined through thefirst control signal C1 input from the controller 168 that the frequencyoffset compensating apparatus shown in FIG. 3 is in the initial mode.The first selector 180 also determines the feedback signal z_(k)^({circumflex u])} (()}û) input from the subtractor 164 as the selectionsignal and then outputs the selection signal if it is determined throughthe first control signal C1 that the frequency offset compensatingapparatus is in the normal mode.

The first delayer 182 delays the selection signal input from the firstselector 180 by a unit block N and then outputs the delayed selectionsignal to the first multiplier 186. Here, the first conjugate calculator184 calculates a conjugate of the selection signal input from the firstselector 180 and then outputs the conjugate of the selection signal tothe first multiplier 186.

The first multiplier 186 multiplies the conjugate of the selectionsignal input from the first conjugate calculator 184 by the delayedselection signal input from the first delayer 182 and then outputs themultiplication result to the first offset calculator 188.

The first offset calculator 188 accumulates the multiplication resultinput from the first multiplier 186 by N_(u)−1 that is one less than thenumber N_(u) of carriers used by the u^(th) user, calculates an angle∠C_(u) of the accumulation result, divides the angle ∠C_(u) by 2π asshown in Equation 1, and outputs the division result as the estimatedfrequency offset {circumflex over (ε)}{circumflex over (ε_(u))}.

The first selector 180, the first delayer 182, the first conjugatecalculator 184, the first multiplier 186, and the first offsetcalculator 188 shown in FIG. 4 are enabled in response to a secondcontrol signal C2 input from the controller 168. Here, in order toperform step 16 of FIG. 1, the controller 168 checks whether apredetermined period of time has elapsed and then outputs the secondcontrol signal C2 in response to the checked result. Here, the elapse ofthe period of time elapses indicates that steps 10, 12, and 14 arerepeated the predetermined number of times. Accordingly, when it isperceived through the second control signal that the predeterminedperiod of time has not elapsed, the first selector 180, the firstdelayer 182, the first conjugate calculator 184, the first multiplexer186, and the first offset calculator 188 are enabled.

In order to perform step 12, the interference estimator 162 estimatesmultiple access interferences (as calculated using Equation 5) whichrepresents the extent to which the reception signals r_(k) ^([1]), . . ., r_(k) ^([u−1]), r_(k) ^([u+1]), . . . and r_(k) ^([U]) for the i^(th)users interfere with the reception signal r_(k) ^([u]) for the u^(th)user, from the reception signals r_(k) ^([1]), . . . , r_(k) ^([u−1]),r_(k) ^([u+1]), . . . and r_(k) ^([U]) for other users, the selectionsignal output from the first selector 180 shown in FIG. 4, and theestimated frequency offset {circumflex over (ε)}{circumflex over(ε_(u))}, and then outputs the estimated multiple access interferencesto the subtractor 164.

To carry out step 14, the subtractor 164 subtracts the estimatedmultiple access interferences from the reception signal r_(k) ^([u])using equation 6 and then outputs the subtraction result as the feedbacksignal

z k ( u ) ⁡ [ u ]to the main frequency offset estimator 160 and the transmission signalestimator 166.

In order to perform step 18, the transmission signal estimator 166estimates a transmission signal from the feedback signal

z k ( u ) ⁡ [ u ]finally input from the subtractor 164 and the estimated frequency offset

ɛ uinput from the main frequency offset estimator 160 using Equation 7 inresponse to the second control signal C2 generated by the controller168, and then outputs the estimated transmission signal

y k [ u ]via an output port OUT1. For example, if the transmission signalestimator 166 perceives through the second control signal C2 that thepredetermined period of time has elapsed, the transmission signalestimator 166 performs an operation to estimate the transmission signal

y k [ u ] .Here, the interference estimator 162 and the subtractor 164 also operatein response to the second control signal C2 generated by the controller168. In other words, if the interference estimator 162 and thesubtractor 164 perceive through the second control signal C2 that thepredetermined period of time has elapsed, they are enabled.

The structure and operation of preferred embodiments of the presentinvention of the transmission signal estimator 166 and the interferenceestimator 162 shown in FIG. 3 will be described with reference to FIGS.5 and 6.

FIG. 5 is a block diagram of a preferred embodiment 166A of the presentinvention of the transmission signal estimator 166 shown in FIG. 3. Thetransmission signal estimator 166A includes a first gain calculator 190,an inverter 192, and a second multiplier 194.

The first gain calculator 190 of the transmission signal estimator 166Ashown in FIG. 5 calculates a gain from the estimated frequency offset{circumflex over (ε)}{circumflex over (ε_(u))} input from the mainfrequency offset estimator 160 using Equation 8 and then outputs thecalculation result as a first gain to the inverter 192.

$\begin{matrix}{{- {\mathbb{e}}^{{- j}\;{x{\lbrack{{\Delta_{u}{({{2{k/N}} - {1/L_{u}}})}} + 1}\rbrack}}}}\frac{L_{u}\sin\;\left( {\pi\;{{\hat{ɛ}}_{u}/L_{u}}} \right)}{{\hat{q}}_{u}{\sin\left( {\pi\;{\hat{ɛ}}_{u}} \right)}}} & (8)\end{matrix}$

Here, the inverter 192 inverts the first gain input from the first gaincalculator 190 and then outputs the inversion result to the secondmultiplier 194. The second multiplier 194 multiply the first gaininverted by the inverter 192 by the feedback signal

z k ( u ) ⁡ [ u ]finally input from the subtractor 164 and then outputs themultiplication result as the estimated transmission signal

y k [ u ] .

The first gain calculator 190, the inverter 192, and the secondmultiplier 194 shown in FIG. 5 are enabled in response to the secondcontrol signal C2 input from the controller 168. For example, if thefirst gain calculator 190, the inverter 192, and the second multiplier194 perceive through the second control signal C2 that the predeterminedperiod of time has elapsed, they are enabled.

FIG. 6 is a block diagram of a preferred embodiment 162A of the presentinvention of the interference estimator 162 shown in FIG. 3. Theinterference estimator 162A includes first, . . . , i^(th), . . . , andU−1^(th) sub frequency offset estimators 200, . . . , 202, . . . , and204, first, . . . , i^(th), . . . , and U−1^(th) extent estimators 210,. . . , 212, . . . , and 214, an adder 216, and a feedback signalgenerator 218.

The i^(th) sub frequency offset estimator 202 of the first, . . . ,i^(th), . . . , and U−1^(th) sub frequency offset estimators 200, . . ., 202, . . . , and 204 selects a feedback signal Z_(k) ^((i)[i]) for thei^(th) user or the reception signal r_(k) ^([i]) for the i^(th) user inresponse to the first control signal C1 input from the controller 168,estimates a frequency offset for the i^(th) other user from the selectedresult, and outputs the estimated frequency offset {circumflex over(ε)}{circumflex over (ε_(i))} to the i^(th) extent estimator 212. Here,the frequency offset is calculated using Equations 1 and 2. In thiscase, i can be substituted for u in Equations 1 and 2.

For example, the first sub frequency offset estimator 200 selects afeedback signal Z_(k) ^((1)[1]) for a first other user or the receptionsignal r_(k) ^([1]) for the first other user in response to the firstcontrol signal C1 input from the controller 168, estimates a frequencyoffset for the first other user from the selected result, and outputsthe estimated frequency offset signal {circumflex over (ε)}{circumflexover (ε₁)} to the first extent estimator 210. The U−1^(th) sub frequencyoffset estimator 204 selects a feedback signal Z_(k) ^((U−1)[U−1]) for aU−1^(th) user or the reception signal r_(k) ^([U−1]) for U−1^(th) otheruser in response to the first control signal C1 input from thecontroller 168, estimates a frequency offset for the U−1^(th) other userfrom the selected result, and outputs the estimated frequency offset{circumflex over (ε)}{circumflex over (ε_(U−1))} to the U−1^(th) extentestimator 214.

FIG. 7 is a block diagram of a preferred embodiment 202A of the presentinvention of the i^(th) sub frequency offset estimator 202 shown in FIG.6. The i^(th) sub frequency offset estimator 202A includes a secondselector 230, a second delayer 232, a second conjugate calculator 234, athird multiplier 236, and a second offset calculator 238.

The second selector 230 shown in FIG. 7 selects one of a feedback signal

z k ( i ) ⁡ [ i ]for the i^(th) other user and the reception signal r_(k) ^([i]) inputfrom the outside in response to the first control signal C1 and thenoutputs the selected result to the second delayer 232 and the secondconjugate calculator 234 and to the i^(th) extent estimator 212 via anoutput port OUT5. For example, if the second selector 230 perceivesthrough the first control signal C1 that the frequency offsetcompensating apparatus is in the initial mode, the second selector 230selects the reception signal r_(k) ^([i]) for the i^(th) other user.However, if the second selector 230 perceives through the first controlsignal C1 that the frequency offset compensating apparatus is in thenormal mode, the second selector 230 selects the feedback signal

z k ( i ) ⁡ [ i ]for the i^(th) other user.

The second delayer 232 delays the selected result input from the secondselector 230 by a unit block N and then outputs the delayed result tothe third multiplier 236. The second conjugate calculator 234 calculatesa conjugate of the selected result input from the second selector 230and then outputs the calculation result to the third multiplier 236. Thethird multiplier 236 multiplies the calculation result input from thesecond conjugate calculator 234 by the delayed result input from thesecond delayer 232 and then outputs the multiplication result to thesecond offset calculator 238.

The second offset calculator 238 accumulates the multiplication resultinput from the third multiplier 236 by N_(i)−1 that is one less than thenumber N_(i) of carriers used by the i^(th) other user, calculates anangle ∠C_(i) of the accumulation result, divides the angle ∠C_(i) by apredetermined number, e.g., 2π, and outputs the division result as theestimated frequency offset {circumflex over (ε)}{circumflex over(ε_(i))} for the i^(th) other user.

The first, . . . , i^(th), . . . , and U−1^(th) extent estimators 210, .. . , 212, . . . , and 214 estimate first, . . . , i^(th), . . . , andU−1^(th) interferences and then output the estimated first, . . . ,i^(th), . . . , and U−1^(th) interferences

z k ( 1 ) ⁡ [ u ] , … ⁢ , z k ( i ) ⁡ [ u ] , … ⁢ , and ⁢ ⁢ z k ( U - 1 ) ⁡ [ u]to the adder 216 and the feedback signal generator 218. For example, thei^(th) extent estimator 212 of the first, . . . , i^(th), . . . , andU−1^(th) extent estimators 210, . . . , 212, . . . , and 214 estimates ai^(th) interference

z k ( i ) ⁡ [ u ]corresponding to the extent to which the reception signal r_(k) ^([i])for the i^(th) other user interferes with the reception signal r_(k)^([u]) for the u^(th) user, from the frequency offset {circumflex over(ε)}{circumflex over (ε_(i))} of the signal r_(k) ^([i]) for the i^(th)other user and the result selected by the i^(th) sub frequency offsetestimator 202, e.g., the selection result output from the secondselector 230 of the i^(th) sub frequency offset estimator 202A shown inFIG. 7 via the output port OUT5, and then outputs the estimation result

z k ( i ) ⁡ [ u ]to the adder 216 and the feedback signal generator 218. The first extentestimator 210 estimates a first interference

z k ( i ) ⁡ [ u ]corresponding to the extent to which the reception signal r_(k) ^([1])for the first other user interfere with the reception signal r_(k)^([u]) for the u^(th) user, from the frequency offset {circumflex over(ε)}{circumflex over (ε₁)} for the first other user and the resultselected by the first sub frequency offset estimator 200, and thenoutputs the estimation result

z k ( i ) ⁡ [ u ]to the adder 216 and the feedback signal generator 218. The U−1^(th)extent estimator 214 estimates a U−1^(th) interference

z k ( U - 1 ) ⁡ [ u ]corresponding to the extent to which a reception signal r_(k) ^([U−1])for a U−1^(th) other user interfere with the reception signal r_(k)^([u]) for the u^(th) user, from a frequency offset {circumflex over(_(U−1))} for the U−1^(th) other user and the result selected by theU−1^(th) sub frequency offset estimator 204, and then outputs theestimation result

z k ( U - 1 ) ⁡ [ u ]to the adder 216 and the feedback signal generator 218.

FIG. 8 is a block diagram of a preferred embodiment 212A of the presentinvention of the i^(th) extent estimator 212 shown in FIG. 6. The i^(th)extent estimator 212A includes a comparator 250, a signal expander andreducer 252, a second gain calculator 254, a third gain calculator 256,fourth and fifth multipliers 258 and 260, and a third selector 262.

The comparator 250 compares the number N_(u) of subcarriers used by theu^(th) user with the number N_(i) of subcarriers used by the i^(th) userand then outputs the comparison result to the third selector 262 and thesignal expander and reducer 252. Here, the signal expander and reducer252 expands or reduces the length of the selection result input from thesecond selector 230 via an input port IN3 in response to the comparisonresult input from the comparator 250. For example, if the signalexpander and reducer 252 perceives through the comparison result inputfrom the comparator 250 that the number N_(i) of subcarriers used by thei^(th) other user is less than the number N_(u) of subcarriers used bythe u^(th) user, the signal expander and reducer 252 expands the lengthof the selection result

z k ( i ) ⁡ [ 1 ]input from the second selector 230 using Equation 9 below and thenoutputs the expansion result to the fourth multiplier 258.

z k ⁢ % ⁢ N i ( i ) ⁡ [ i ] ( 9 )

However, if the signal expander and reducer 252 perceives through thecomparison result input from the comparator 250 that the number N_(i) ofsubcarriers used by the i^(th) other user is greater than the numberN_(u) of subcarriers used by the u^(th) user, the signal expander andreducer 252 reduces the length of the selection result

z k ( i ) ⁡ [ i ]input from the second selector 230 using Equation 10 below and thenoutputs the reduction result to the fifth multiplier 260.

∑ 1 = 0 M iu - 1 ⁢ ⅇ j ⁢ ⁢ 2 ⁢ π ⁡ ( Δ iu + ɛ i ⋀ ) ⁢ 1 / L u ⁢ z k + 1 ⁢ N u (i ) ⁡ [ i ] ( 10 )

The second gain calculator 254 calculates a gain from the frequencyoffset {circumflex over (ε)}{circumflex over (ε_(i))} for the i^(th)other user and input from the i^(th) sub frequency offset estimator 202using Equation 11 below and then outputs the calculation result as asecond gain to the fourth multiplier 258.

$\begin{matrix}{{\mathbb{e}}^{j\;{\pi{\lbrack{{\Delta_{iu}{({{2{k/N}} - {1/L_{u}}})}} + {\overset{\bigwedge}{ɛ_{i}}{({{1/L_{1}} - {1/L_{u}}})}}}\rbrack}}}\frac{q_{i}{\sin\left( {\pi\;{\overset{\bigwedge}{ɛ_{i}}/L_{i}}} \right)}}{L_{i}{\sin\left\lbrack {{\pi\left( {\Delta_{iu} + \overset{\bigwedge}{ɛ_{i}}} \right)}/L_{u}} \right\rbrack}}} & (11)\end{matrix}$

The third gain calculator 256 calculates a gain from the frequencyoffset {circumflex over (ε)}{circumflex over (ε_(i))} for the i^(th)other user and input from the i^(th) sub frequency offset estimator 202using Equation 12 below and then outputs the calculation result as athird gain to the fifth multiplier 260.

$\begin{matrix}{{\mathbb{e}}^{j\;{\pi{\lbrack{\Delta_{iu}{({{2{k/N}} - {1/L_{i}}})}}\rbrack}}}\frac{q_{i}{\sin\left( {\pi\;{\overset{\bigwedge}{ɛ_{i}}/L_{i}}} \right)}}{L_{i}{\sin\left\lbrack {{\pi\left( {\Delta_{iu} + \overset{\bigwedge}{ɛ_{i}}} \right)}/L_{i}} \right\rbrack}}} & (12)\end{matrix}$

The fourth multiplier 258 multiplies the expansion result of the lengthof

z k ( i ) ⁡ [ i ]input from the signal expander and reducer 252 by the second gain inputfrom the second gain calculator 254 and then outputs the multiplicationresult to the third selector 262. The fifth multiplier 260 multipliesthe reduction result of the length of

z k ( i ) ⁡ [ i ]input from the signal expander and reducer 252 by the third gain inputfrom the third gain calculator 256 and then outputs the multiplicationresult to the third selector 262.

The third selector 262 selects one of the multiplication results inputfrom the fourth and fifth multipliers 258 and 260 in response to thecomparison result input from the comparator 250 and then outputs theselection result as the i^(th) interference

z k ( i ) ⁡ [ i ] .For example, if the third selector 262 perceives through the comparisonresult input from the comparator 250 that the number N_(i) ofsubcarriers used by the i^(th) other user is less than the number N_(u)of subcarriers used by the u^(th) user, the third selector 262 selectsthe multiplication result obtained by the fourth multiplier 258.However, if the third selector 262 perceives through the comparisonresult input from the comparator 250 that the number N_(i) ofsubcarriers used by the i^(th) other user is greater than the numberN_(u) of subcarriers used by the u^(th) user, the third selector 262selects the multiplication result obtained by the fifth multiplier 260.

The adder 216 shown in FIG. 6 adds the first, . . . , i^(th), . . . ,and U−1^(th) interferences output from the first, . . . , i^(th), . . ., and U−1^(th) extent estimators 210, . . . , 212, . . . , and 214 andthen output the addition result as the interference expressed byEquation 5 via an output port OUT4.

The feedback signal generator 218 shown in FIG. 6 generates feedbacksignals z_(k) ^((1)[1]), . . . , z_(k) ^((i)[i]), . . . , and z_(k)^((U−1)[U−1]) used in the first, . . . , i^(th), . . . , and U−1^(th)sub frequency offset estimators 200, . . . , 202, . . . , and 204 fromthe first gain input via an input port IN1, the selection signal inputvia an input port IN2, the first, . . . , i^(th), . . . , and U−1^(th)interferences input from the first, . . . , i^(th), . . . , and U−1^(th)extent estimators 210, . . . , 212, . . . , and 214, and the receptionsignals r_(k) ^([1]), . . . , r_(k) ^([i]), . . . , and r_(k) ^([U−1])for the other users.

FIG. 9 is a block diagram of a preferred embodiment 218A of the presentinvention of the feedback signal generator 218 shown in FIG. 6. Thefeedback signal generator 218A includes first, . . . , i^(th), . . . ,and U−1^(th) subtractors 280, . . . , 282, . . . , and 284 and a sixthmultiplier 286.

According to an embodiment of the present invention, the feedback signalgenerator 218A shown in FIG. 9 can include the sixth multiplier 286 toreceive the first gain output from the first gain calculator 190 of thetransmission signal estimator 166A shown in FIG. 5 via an output portOUT3, via an input port IN4, to receive the selection signal output fromthe first selector 180 of the main frequency offset estimator 160A viathe output port OUT2, via an input port IN5, to multiply the selectionsignal by the first gain, and to output the multiplication result toeach of the first, . . . , i^(th), . . . , and U−1^(th) subtractors 280,. . . , 282, . . . , and 284.

According to another embodiment of the present invention, the feedbacksignal generator 218A shown in FIG. 9 can further include a gaingenerator (not shown). In this case, the gain generator can receive theestimated frequency offset {circumflex over (ε)}{circumflex over(ε_(u))} input from the main frequency offset estimator 160 via theinput port IN1 to calculate a first gain using Equation 8. Here, thesixth multiplier 286 receives the first gain generated by the gaingenerator instead of receiving the first gain from the transmissionsignal estimator 166A via the input port IN4 and then multiplies thefirst gain by the selection signal input via the input port IN5.

The first subtractor 280 subtracts interferences

z k ( 2 ) ⁢ ( u ) , ⁢ … ⁢ , z k ( i - 1 ) ⁡ [ u ] , z k ( i ) ⁡ [ u ] , z k (i + 1 ) ⁡ [ u ] ⁢ ⁢ … ⁢ , z k ( U - 2 ) ⁡ [ u ] , and ⁢ ⁢ z k ( U - 1 ) ⁡ [ u ]of the second through U−1^(th) interferences and the multiplicationresult obtained by the sixth multiplier 286 from the reception signalr_(k) ^([1]) for the first other user and then outputs the subtractionresult as the feedback signal z_(k) ^((1)[l]) used in the first subfrequency offset estimator 200 via an output port OUT6.

The i^(th) subtractor 282 of the first, . . . , i^(th), . . . , andU−1^(th) subtractors 280, . . . , 282, . . . , and 284 subtractsinterferences

z k ( 1 ) ⁡ [ u ] , z k ( 2 ) ⁢ ( u ) ⁢ ⁢ … ⁢ , z k ( i - 1 ) ⁡ [ u ] , z k (i + 1 ) ⁡ [ u ] ⁢ ⁢ … ⁢ , z k ( U - 2 ) ⁡ [ u ] , and ⁢ ⁢ z k ( U - 1 ) ⁡ [ u ]of the first through U−1^(th) interferences from which the i^(th)interference is excluded and the multiplication result obtained by thesixth multiplier 286 from the reception signal r_(k) ^([i]) for thei^(th) other user and then outputs the subtraction result as thefeedback signal z_(k) ^((i)[i]) used in the i^(th) sub frequency offsetestimator 202 via an output port OUT7.

The U−1^(th) subtractor 284 subtracts interferences

z k ( 1 ) ⁡ [ u ] , z k ( 2 ) ⁢ ( u ) ⁢ ⁢ … ⁢ , z k ( i - 1 ) ⁡ [ u ] , z k (i ) ⁡ [ u ] , z k ( i + 1 ) ⁡ [ u ] , ⁢ … ⁢ , and ⁢ ⁢ z k ( U - 2 ) ⁡ [ u ]of the first through U−2^(th) interferences and the multiplicationresult obtained by the sixth multiplier 286 from the reception signalr_(k) ^([U−1]) for the U−1^(th) other user and then outputs thesubtraction result as the feedback signal z_(k) ^((U−1)[U−1]) used inthe U−1^(th) sub frequency offset estimator 204 via an output port OUT8.

In order to help understand the method and apparatus for compensatingfor frequency offset in an IFDMA according to the present invention, letus assume that U is 8, each of the users uses 32 subcarriers, each ofthe subcarriers uses quadrature phase shift keying (QPSK), the frequencyoffsets of the users are the same, and additive white Gaussian noise(AWGN) is input via channels.

FIG. 10 illustrates the constellation of an ideal reception signal thatdoes not have a frequency offset where the vertical and horizontal axesdenote quadrature Q and in-phase I, respectively. Referring to FIG. 10,spots [(−1, −1), (1, −1), (−1, 1), and (1, 1)] indicating receptionsignals on coordinates of I and Q are circularly spread due to theeffect of noise input via channels.

FIG. 11 is a graph for illustrating the constellation of a receptionsignal when a frequency offset accounts for 3% of the distance amongsubcarriers, and FIG. 12 illustrates the constellation of a receptionsignal when the method and apparatus according to the present inventionare applied to constellation shown in FIG. 11.

Referring to FIG. 11, the constellation is circular due to the effect ofthe frequency offset and interferences among users. When the frequencyoffset compensating method and apparatus according to the presentinvention are applied to the circular constellation, the constellationis spread more than the constellation shown in FIG. 10 but does not showthe same circular constellation as in FIG. 12.

The relationship among the predetermined number, SNR, and bit error rate(BER) will be described below.

FIG. 13 is a graph for illustrating a SNR versus the frequency offsetwhere the horizontal and vertical axes denote the frequency offset andSNR, respectively. FIG. 14 is a graph for illustrating variations in BERversus the frequency offset where the horizontal and vertical axesdenote the frequency offset and BER, respectively.

In FIGS. 13 and 14, ‘No OP’ represents an SNR when the frequency offsetis not compensated for, ‘Iter 0’ represents an SNR when steps 10, 12,and 14 are not repeated, ‘Iter 1’ represents an SNR when thepredetermined number of times is 1, ‘Iter 2’ represents an SNR when thepredetermined number of times is 2, ‘Iter 3’ represents an SNR when thepredetermined number of times is 3, ‘Iter 4’ represents an SNR when thepredetermined number of times is 4, and ‘Iter 5’ represents an SNR whenthe predetermined number of times is 5.

As can be seen in FIG. 13, the SNR sharply improves with an increase inthe predetermined number of repetitions. In particular, comparing ‘NoOp’ and ‘Iter 1’, the SNR is increased by 12 dB by compensating for thefrequency offset when the frequency offset accounts for 10% of thedistance among the subcarriers. Accordingly, if the frequency offset islarge, the SNR can be improved with an increase in the predeterminednumber of times. As can be seen in FIG. 14, if the frequency offsetbecomes large, the predetermined number of repetitions should beincreased to obtain a BER of about 10⁻⁶.

As described above, unlike the prior art using analog RF/IF ports forremoving a frequency offset, a method and apparatus for compensating forthe frequency offset in an IFDMA according to the present invention canremove the amplitude and phase distortions of a reception signal causedby an existing frequency offset and interferences among users in a baseband instead of removing the frequency offset. Therefore, cost fordesigning and realizing circuits can be reduced and the frequency offsetcan be compensated for before estimating the characteristics ofchannels.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those of ordinary skill in the art that various changes in form anddetails may be made therein without departing from the spirit and scopeof the present invention as defined by the following claims.

1. A method of compensating for a frequency offset between atransmission signal and a reception signal for a u^(th) user (1≦u≦U,where U denotes the number of users) in an interleaved frequencydivision multiple access (IFDMA) system, the method comprising the stepsof: (a) estimating the frequency offset from a selection signal in theIFDMA system that is determined as the reception signal for the u^(th)user in an initial mode and as a feedback signal in a normal mode; (b)estimating multiple access interferences representing an extent to whichreception signals for i^(th) other users (1≦i≦U−1) at the same timeinterfere with the reception signal for the u^(th) user; (c) subtractingthe estimated multiple access interferences from the reception signalfor the u^(th) user and determining the subtraction result as thefeedback signal; (d) determining whether steps (a), (b), and (c) havebeen repeated a predetermined number of times, and if it is determinedthat steps (a), (b), and (c) have not been repeated the predeterminednumber of times, going back to step (a); and (e) if it is determinedthat steps (a), (b), and (c) have been repeated the predetermined numberof times, estimating the transmission signal for the u^(th) user usingthe feedback signal finally determined in step (c) and the estimatedfrequency offset, wherein in step (a), the frequency offset is estimatedfrom the selection signal using the following equation:$\hat{ɛ_{u}} = \frac{\angle\; C_{u}}{2\pi}$ where {circumflex over(ε)}_(u) denotes the frequency offset,${{\hat{ɛ_{u}}} < \frac{1}{2}},{\angle\; C_{u}},$ denotes the angle ofa correlation value ∑ k = 1 N u - 1 ⁢ * , N_(u) denotes the number ofcarriers used by the u^(th) user, k (k=0, 1, . . . , and N_(u)−1)denotes the order of the reception signal for the u^(th) user among aplurality of signals contained in a frame, z k ( u ) ⁡ [ u ] denotes theselection signal, N denotes the number of chips constituting a block, zk + N ( u ) ⁡ [ u ] denotes the result obtained by delaying the selectionsignal z k ( u ) ⁡ [ u ] by N, and ( z k ( u ) ⁡ [ u ] ) * denotes aconjugate of the selection signal z k ( u ) ⁡ [ u ] .
 2. The method ofclaim 1, wherein in step (b), the extent z k ( i ) ⁡ [ u ] to which thei^(th) other user among other users interfere with the u^(th) user isestimated using the following equation: z k ( i ) ⁡ [ u ] = { ⅇ j ⁢ ⁢ π ⁡ [Δ i ⁢ ⁢ u ⁡ ( 2 ⁢ k / N - 1 / L u ) + ɛ i ^ ⁡ ( 1 / L i - 1 / L u ) ] ⁢ q i ⁢sin ⁡ ( π ⁢ ɛ i ^ / L i ) L i ⁢ sin ⁡ [ π ⁡ ( Δ i ⁢ ⁢ u + ɛ i ^ ) / L u ] · z k⁢% ⁢ N i ( i ) ⁡ [ i ] , for ⁢ ⁢ N i ≤ N u ⅇ j ⁢ ⁢ π ⁡ [ Δ i ⁢ ⁢ u ⁡ ( 2 ⁢ k / N - 1/ L i ) ] ⁢ q i ⁢ sin ⁡ ( π ⁢ ɛ i ^ / L i ) L i ⁢ sin ⁡ [ π ⁡ ( Δ i ⁢ ⁢ u + ɛ i ^) / L i ] · ∑ 1 = 0 M i ⁢ ⁢ u - 1 ⁢ ⅇ j2 ⁢ ⁢ π ⁡ ( Δ i ⁢ ⁢ u + ɛ i ^ ) ⁢ 1 / L u ⁢z k + 1 ⁢ N u ( i ) ⁡ [ i ] , for ⁢ ⁢ N i > N u where k % N_(i) denotes theremainder when k is divided by N_(i), N_(i) denotes the number ofcarriers used by the i^(th) user, L_(i) denotes the number of times usersymbols are repeated in a transmitter for the i^(th) user, L_(u) denotesthe number of times user symbols are repeated in a transmitter for theu^(th) user, Δ_(iu) denotes n_(i)−n_(u), n_(i) denotes a frequencyoffset assigned to the i^(th) user, n_(u) denotes a frequency offsetassigned to the u^(th) user, q_(i) denotes an initial phase offset in ani^(th) block, and$M_{i\; u} = {\frac{N_{i}}{N_{u}} = {\frac{L_{u}}{L_{i}}.}}$
 3. Themethod of claim 2, wherein in step (c), the estimated interference ∑ i =1 , i ≠ u U ⁢ z k ( i ) ⁡ [ u ] is subtracted from the reception signalfor the u^(th) user using the following equation: z k ( u ) ⁡ [ u ] = r k[ u ] - ∑ i = 1 , i ≠ u U ⁢ z k ( i ) ⁡ [ u ] where z k ( u ) ⁡ [ u ] isthe feedback signal and r_(k) ^([u]) denotes the reception signal forthe u^(th) user.
 4. The method of claim 3, wherein the predeterminednumber of times is determined in proportion to a rate by which asignal-to-noise ratio decreases.
 5. The method of claim 4, wherein instep (e), if it is determined that steps (a), (b), and (c) have beenrepeated the predetermined number of times, the transmission signaly_(k) ^([u]) for the u^(th) user is estimated using the feedback signalz k ( u ) ⁡ [ u ] finally determined in step (c) and the estimatedfrequency offset {circumflex over (ε)}_(u) according to the followingequation: y k [ u ] = ⅇ - j ⁢ ⁢ π ⁡ [ Δ iu ⁡ ( 2 ⁢ k / N - 1 / L u ) + 1 ] ⁢ Lu ⁢ sin ⁡ ( π ⁢ ⁢ ɛ ^ u / L u ) q ^ u ⁢ sin ⁡ ( π ⁢ ⁢ ɛ u ^ ) ⁢ z k ( u ) ⁡ [ u ]where y_(k) ^([u]) denotes the estimated transmission signal for theu^(th) user and q_(u) denotes an initial phase offset in an u^(th)block.
 6. An apparatus for compensating for a frequency offset between atransmission signal and a reception signal for an u^(th) user (1≦u≦U,where U denotes the number of users) in an interleaved frequencydivision multiple access (IFDMA) system, the apparatus comprising: amain frequency offset estimator for determining the reception signal forthe u^(th) user or a feedback signal as a selection signal in responseto a first control signal in the IFDMA system, estimating the frequencyoffset from the selection signal, and outputting the estimated frequencyoffset; an extent estimator for estimating multiple access interferencesrepresenting an extent to which reception signals for from i^(th) otherusers (1≦i≦U−1) interfere with the reception signal for the u^(th) user,from the reception signals for the i^(th) other users, the selectionsignal, and the estimated frequency offset, and outputting the estimatedmultiple access interferences; a subtractor for subtracting theestimated interferences from the reception signal for the u^(th) userand outputting the subtraction result as the feedback signal; acontroller for generating the first control signal in response to theresult obtained by analyzing the state of the apparatus for compensatingfor the frequency offset, checking whether a predetermined period oftime has elapsed, and outputting a second control signal in response tothe checked result; and a transmission signal estimator for estimatingthe transmission signal for the u^(th) user from the feedback signalfinally input from the subtractor and the estimated frequency offset inresponse to the second control signal and outputting the estimatedtransmission signal, wherein the main frequency offset estimator, theextent estimator, and the subtractor are enabled in response to thesecond control signal, wherein the main frequency offset estimatorcomprises: a first selector for selecting one of the feedback signalsinput from the subtractor and the reception signal for the u^(th) userinput from the outside in response to the first control signal andoutputting the selection result as the selection signal; a first delayerfor delays the selection signal input from the first selector by a unitblock and outputting the delayed selection signal; a first conjugatecalculator for calculating a conjugate of the selection signal inputfrom the first selector and outputting the calculated conjugate of theselection signal; a first multiplier for multiplying the conjugate ofthe selection signal input from the first conjugating calculator by thedelayed selection signal input from the first delayer and outputting themultiplication result; and a first offset calculator for accumulatingthe multiplication result input from the first multiplier by N_(u)−1that is one less than the number N_(u) of carriers used by the u^(th)user, calculating an angle of the accumulation result, divides the angleby a predetermined number, and outputting the division result as theestimated frequency offset, wherein the first selector, the firstdelayer, the first conjugate calculator, the first multiplier, and thefirst frequency offset are enabled in response to the second controlsignal.
 7. The apparatus of claim 6, wherein the transmission signalestimator comprises: a first gain calculator for calculating a gain fromthe estimated frequency offset input from the main frequency offsetestimator using equation below and outputting the calculation result asa first gain:${- {\mathbb{e}}^{{- j}\;{\pi\;\lbrack{{\Delta_{iu}{({{2{k/N}} - {1/L_{u}}})}} + 1}\rbrack}}}\frac{L_{u}{\sin\left( {\pi\mspace{11mu}{{\hat{ɛ}}_{u}/L_{u}}} \right)}}{q_{u}{\sin\left( {\pi\mspace{11mu}\hat{ɛ_{u}}} \right)}}$where k (k=0, 1, . . . , and N_(u)−1) denotes the order of thecorresponding reception signal for the u^(th) user among a plurality ofreception signals contained in a frame, N denotes the number of chipsconstituting a block, L_(u) denotes the number of times user symbols arerepeated in a transmitter for the u^(th) user, {circumflex over (ε)}_(u)denotes the frequency offset for the u^(th) user, Δ_(iu) denotesn_(i)−n_(u), n_(i) denotes a frequency offset assigned to the i^(th)user, n_(u) denotes a frequency offset assigned to the u^(th) user, andq_(u) denotes an initial phase offset in an u^(th) block; an inverterfor inverting the first gain; and a second multiplier for multiplyingthe feedback signal finally input from the subtractor by the invertedfirst gain and outputting the multiplication result as the estimatedtransmission signal, wherein the first gain calculator, the inverter,and the second multiplier are enabled in response to the second controlsignal.
 8. The apparatus of claim 7, wherein the extent estimatorcomprises: first through U−1^(th) sub frequency offset estimators; firstthrough U−1^(th) extent estimators that estimate the first throughU−1^(th) interferences; an adder; and a feedback signal generator,wherein the i^(th) sub frequency offset estimator selects the feedbacksignal for the i^(th) user or the reception signal for the i^(th) otheruser in response to the first control signal and then estimates afrequency offset for the i^(th) other user from the selection result,the i^(th) extent estimator estimates the i^(th) interferencecorresponding to the extent to which the reception signal for the i^(th)other user interfere with the reception signal for the u^(th) user, fromthe frequency offset for the i^(th) other user and the selection resultselected by the i^(th) sub frequency offset estimator, the adder addsthe first through U−1^(th) interferences and then outputs the additionresult as the interference, and the feedback signal generator generatesfeedback signals used in the first through U−1^(th) sub frequency offsetestimators from the first gain, the selection signal, the first throughU−1^(th) interferences, and the reception signals for the other users.9. The apparatus of claim 8, wherein the i^(th) sub frequency offsetestimator comprises: a second selector for selecting one of the feedbacksignal for the i^(th) other user and the reception signal for the i^(th)other user input from the outside in response to the first controlsignal and outputting the selection result; a second delayer for delaysthe selection result input from the second selector by the unit blockand then outputting the delayed result; a second conjugate calculatorfor calculating a conjugate of the selection result input from thesecond selector and then outputting the calculation result; a thirdmultiplier for multiplying the calculation result input from the secondconjugate calculator by the delayed result input from the second delayerand then outputting the multiplication result; and a second offsetcalculator for accumulating the multiplication result input from thethird multiplier by N_(i)−1 that is one less than the number N_(i) ofcarriers used by the i^(th) other user, calculating an angle of theaccumulation result, dividing the angle by a predetermined number, andoutputting the division result as the estimated frequency offset for thei^(th) other user.
 10. The apparatus of claim 8, wherein the i^(th)extent estimator comprises: a comparator for comparing the number N_(u)of subcarriers used by the u^(th) user with the number N_(i) ofsubcarriers used by the i^(th) other user and then outputting thecomparison result; a signal expander and reducer for expanding orreducing the length of the selection result z k ( i ) ⁡ [ i ] input fromthe second selector in response to the comparison result input from thecomparator according to equation below and then outputting the expansionor reduction result: z k ⁢ % ⁢ N i ( i ) ⁡ [ i ] for ⁢ ⁢ N i ≤ N u ∑ l = 0 Miu - 1 ⁢ ⅇ j ⁢ ⁢ 2 ⁢ ⁢ π ( Δ iu + ɛ i ) ^ ⁢ 1 / L u ⁢ z k + 1 ⁢ N u ( i ) ⁡ [ i ]for ⁢ ⁢ N i > N u where {circumflex over (ε)}_(i) denotes the frequencyoffset for the i^(th) user; a second gain calculator for calculating again from the frequency offset, for the i^(th) other user, input fromthe i^(th) sub frequency offset estimator according to equation belowand then outputting the calculation result as a second gain:${\mathbb{e}}^{{j\;{\pi\;\lbrack{{\Delta_{iu}{({{2{k/N}} - {1/L_{u}}})}} + {\hat{ɛ_{i}(}{1/L_{i}}} - {1/L_{u}}})}}\rbrack}\frac{q_{i}{\sin\left( {\pi\mspace{11mu}{\hat{ɛ_{i}}/L_{i}}} \right)}}{L_{i}{\sin\left\lbrack {{\pi\left( {\Delta_{iu} + \hat{ɛ_{i}}} \right)}/L_{u}} \right\rbrack}}$where L_(i) denotes the number of times user symbols are repeated in atransmitter for the i^(th) user and q_(i) denotes an initial phaseoffset in an i^(th) block; a third gain calculator for calculating again from the frequency offset, for the i^(th) other user, input fromthe i^(th) sub frequency offset estimator according to equation belowand then outputting the calculation result as a third gain:${\mathbb{e}}^{j\;{\pi\;\lbrack{\Delta_{iu}{({{2{k/N}} - {1/L_{i}}})}}\rbrack}}\frac{q_{i}{\sin\left( {\pi\mspace{11mu}{\hat{ɛ_{i}}/L_{i}}} \right)}}{L_{i}{\sin\left\lbrack {{\pi\left( {\Delta_{iu} + \hat{ɛ_{i}}} \right)}/L_{i}} \right\rbrack}}$${{{where}\mspace{14mu} M_{iu}} = {\frac{N_{i}}{N_{u}} = \frac{L_{u}}{L_{i}}}};$a fourth multiplier for multiplying the expansion result input from thesignal expander and reducer by the second gain and then outputting themultiplication result; a fifth multiplier for multiplying the reductionresult input from the signal expander and reducer by the third gain andthen outputting the multiplication result; and a third selector forselecting one of the multiplication results input from the fourth andfifth multipliers in response to the comparison result input from thecomparator and then outputting the selection result as the i^(th)interference.
 11. The apparatus of claim 8, wherein the feedback signalgenerator comprises: a sixth multiplier for multiplying the first gaininput from the first gain calculator by the selection signal input fromthe first selector and then outputting the multiplication result; andfirst through U−1^(th) subtractors, wherein the i^(th) subtractorsubtracts the interferences of the first through U−1^(th) interferencesfrom which the i^(th) interference is excluded, and the multiplicationresult of the sixth multiplier from the reception signal for the i^(th)other user and then outputs the subtraction result as the feedbacksignal used in the i^(th) sub frequency offset estimator.